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As far as I'm aware, that doesn't ensure that Module::Build is upgraded, it just crashes the installer with an obscure error message that will be buried somewhere in the middle of 3000 lines of CPAN.pm output.

If you are doing a one-off install and running Build.PL directly, and you understand error messages, then maybe in that small case it's good enough. but of course almost nobody does that.

As for the build_requires dependency, the Build.PL needs to be run in order to confirm that

I don't have a distribution with a custom Module::Build subclass where the build_requires trick won't work, but if the use line doesn't, how about printing the same error message? M::B and CPAN.pm communicate somehow, so I see no reason why following that protocol won't make this scheme work.

CPAN.pm and CPANPLUS both somehow detect when a distribution has unfulfilled dependencies. Both offer to install those dependencies.

Both MakeMaker and Module::Buildsomehow indicate unfulfilled dependencies to the installer. I don't know if it's screenscraping or an API or whatever. I skimmed CPAN.pm this morning, but couldn't find it in two minutes.

Now if there's an API or if there's a particular error message being scraped and detected and

I am suggesting that, in those cases where the installation program depends on a specific minimum version of the installer module (and in several years of maintaining a couple of dozen of publicly available modules, I can think of one case where this was necessary), the author should do precisely that.

There already exists a perfectly good mechanism to mark and install dependencies through both of the installer shells. Why complicate the process?

I am suggesting that, in those cases where the installation program depends on a specific minimum version of the installer module (and in several years of maintaining a couple of dozen of publicly available modules, I can think of one case where this was necessary), the author should do precisely that.

That is certainly an acceptable partial solution for the cases where dependencies are static, and one that individual authors can use today.

I mean, "Authors of distributions that rely on a specific version of the bundling module should be able to mark that dependency in such a way that the installer can detect that dependency immediately." Whether that means adding a new entry in META.yml or running a little bit of code at the start of Build.PL or Makefile.PL, I don't particularly care.

The latter seems easiest.

The point is, I believe you can run just enough of the bundling program to flag an unmet dependency on the bundling module that the installer shell should go off to resolve that dependency immediately. You don't have to continue running the program at all at that point.

I don't think getting this done means that Module::Install has to go away; I've always seen its biggest value for distributing full applications with their dependencies to users who can't, won't, or shouldn't install it themselves from the CPAN.

I can see a number of ways in which it could be done, a few of which you have described.

It's just that the situation remains that despite what should be possible, it still hasn't been written, and what I try to keep pointing out is that there needs to be, and that whatever the solution is, it needs to be completed and working before we can consider M:B to be stable and suitable for the core.